As electronic applications become increasingly complex great demands are placed on the switches used therein. In the past relays were employed for switching applications. Monolithic switches replaced relays, increasing the number of frequencies within which the switches have a substantially constant gain. These frequencies make up the usable bandwidth of the switch. In order to increase the bandwidth further and to decrease the operating voltage of the monolithic switches, finer manufacturing processes are being used.
The upper limit of the bandwidth of switches increases as the process used becomes more fine. However, many switches require a current source feeding into the signal path. This current source must have a high impedance so as not to degrade the signal in the signal path. A resistor would not be employed as the current source for demanding applications as it would cause distortion. An active component must be used. This introduces a resistor-capacitor network having a pole-zero combination. To keep the pole-zero combination above the otherwise usable bandwidth, a high impedance, high unity gain frequency, f.tau., transistor may be employed as the current source. However, these transistors are expensive to manufacture.
Another problem encountered with some switches results from employing bipolar junction transistors for which it is desirable that the base-emitter voltages, V.sub.BE 's, be substantially similar while the collector-emitter voltages (V.sub.CE 's) of those transistors are substantially dissimilar. What constitutes substantial similarity and dissimilarity will depend on the tolerance specifications of the DC offset of the switch. Typical matching techniques for producing substantially similar V.sub.BE 's employ substantially similar currents flowing in the collectors of each transistor. However, where the transistors are operated at substantially dissimilar V.sub.CE 's, the V.sub.BE 's will not necessarily be the same due to base-width modulation effects. The Early voltage, V.sub.A, of the transistors is a measure of these effects. The Early voltage of a transistor is not easily controllable during the manufacturing process, becoming lower, therefore having more effect, as the process becomes increasingly fine. Variations in the Early voltage result in unpredictable values of output offset for the switch. The output offset is the DC voltage difference between the input and the output of the switch. This limits the applications where the switch may be used.
Furthermore, it is sometimes required to indicate when a monolithic switch is on. Typically indications are given of when a particular switch has been selected for operation, i.e. enabled, however this indication only represents a selection of a particular switch or the state of the control logic. It does not represent the actual functioning of the switch once enabled.